Totally irrelevant.
You must not be working for the marketing department...
Most aircraft sales/marketing brochures show the TAS instead of IAS because the TAS is invariably higher. Only when you read the fine print do you realize that that TAS is obtained at some ridiculously high altitude, which nobody flies at in real life, for instance due to oxygen/airspace issues.
Anyway, FlyingLapinou, I can't believe you are not allowed to use some sort of dedicated flight computer. Whether that's a mechanical one or an electronic one doesn't matter. But they are specifically designed for calculations like this, plus things like fuel volume/mass conversions, working out your GS and track based on TAS, heading and wind, and so on and so forth. (I can imagine that it's not allowed for Aircraft Technical, but for a subject like Navigation...
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With such a flight computer, at the PPL level you can forget about all the math that people showed you in this thread. As Peter said, you simply line up the numbers that you know, and read off the number you are looking for. It really is that simple.
And why would I want to know my TAS?
For the PPL exam you need to be able to calculate TAS based on IAS, temperature and altitude, and then you use that TAS in combination with the forecasted wind to calculate GS and heading. Although you won't find any Mach numbers on the PPL exam, just knots.
After the PPL you will find that the difference between TAS and IAS, at typical PPL flight altitudes, is less than the error in the wind forecast, so you can save yourself the trouble. Although "trouble" is a big word for having to line up two numbers, and reading off a third.